How to Decode Partially Double Serialized JSON String Using 'Codable' Protocol

How can I decode partially double serialized json string using `Codable` protocol?

If you can't fix your double encoded json you can provide your own custom decoder method to your PerfectFamily class but I recommend using a struct:

---

struct Person: Codable {
    let name: String
    let hobby: String
}

---

struct PerfectFamily: Codable {
    let person: Person
    init(from decoder: Decoder) throws {
        let container = try decoder.singleValueContainer()
        let person = try container.decode([String: String].self)["person"] ?? ""
        self.person = try JSONDecoder().decode(Person.self, from: Data(person.utf8))
    }
}

---

let json = "{\"person\":\"{\\\"name\\\":\\\"Mike\\\",\\\"hobby\\\":\\\"fishing\\\"}\"}"

do {
    let person = try JSONDecoder().decode(PerfectFamily.self, from: Data(json.utf8)).person
    print(person)   // "Person(name: "Mike", hobby: "fishing")\n"
} catch {
    print(error)
}

How do I decode JSON in Swift where it's an array with double-nested items?

You can decode that JSON without having to introduce intermediate structs while keeping type safety by decoding the outer Dictionary whose only key is data as a nested Dictionary of type [String:[String:[Dog]]], which is pretty messy, but works since you only have 2 nested layers and single keys in the outer dictionaries.

struct Dog: Codable {
    let name:String
}

struct Result: Codable {
    let dogs1: [Dog]
    let dogs2: [Dog]

    enum DogJSONErrors: String, Error {
        case invalidNumberOfContainers
        case noChildrenContainer
    }

    init(from decoder: Decoder) throws {
        var containersArray = try decoder.unkeyedContainer()
        guard containersArray.count == 2 else { throw DogJSONErrors.invalidNumberOfContainers}
        let dogsContainer1 = try containersArray.decode([String:[String:[Dog]]].self)
        let dogsContainer2 = try containersArray.decode([String:[String:[Dog]]].self)
        guard let dogs1 = dogsContainer1["data"]?["children"], let dogs2 = dogsContainer2["data"]?["children"] else { throw DogJSONErrors.noChildrenContainer}
        self.dogs1 = dogs1
        self.dogs2 = dogs2
    }
}

Then you can simply decode a Result instance like this:

do {
    let dogResults = try JSONDecoder().decode(Result.self, from: dogsJSONString.data(using: .utf8)!)
    print(dogResults.dogs1,dogResults.dogs2)
} catch {
    print(error)
}

How can I implement polymorphic decoding of JSON data in Swift 4?

I recommend you to be judicious on the use of Codable. If you only want to decode a type from JSON and not encode it, conforming it to Decodable alone is enough. And since you have already discovered that you need to decode it manually (via a custom implementation of init(from decoder: Decoder)), the question becomes: what is the least painful way to do it?

First, the data model. Note that ViewLayoutSectionItemable and its adopters do not conform to Decodable:

enum ItemType: String, Decodable {
    case foo
    case bar
}

protocol ViewLayoutSectionItemable {
    var id: Int { get }
    var itemType: ItemType { get }
    var title: String { get set }
    var imageURL: URL { get set }
}

struct Foo: ViewLayoutSectionItemable {
    let id: Int
    let itemType: ItemType
    var title: String
    var imageURL: URL
    // Custom properties of Foo
    var audioURL: URL
}

struct Bar: ViewLayoutSectionItemable {
    let id: Int
    let itemType: ItemType
    var title: String
    var imageURL: URL
    // Custom properties of Bar
    var videoURL: URL
    var director: String
}

Next, here's how we will decode the JSON:

struct Sections: Decodable {
    var sections: [ViewLayoutSection]
}

struct ViewLayoutSection: Decodable {
    var title: String = ""
    var sectionLayoutType: String
    var sectionItems: [ViewLayoutSectionItemable] = []

    // This struct use snake_case to match the JSON so we don't have to provide a custom
    // CodingKeys enum. And since it's private, outside code will never see it
    private struct GenericItem: Decodable {
        let id: Int
        let item_type: ItemType
        var title: String
        var feature_image_url: URL
        // Custom properties of all possible types. Note that they are all optionals
        var audio_url: URL?
        var video_url: URL?
        var director: String?
    }

    private enum CodingKeys: String, CodingKey {
        case title
        case sectionLayoutType = "section_layout_type"
        case sectionItems = "section_items"
    }

    public init(from decoder: Decoder) throws {
        let container     = try decoder.container(keyedBy: CodingKeys.self)
        title             = try container.decode(String.self, forKey: .title)
        sectionLayoutType = try container.decode(String.self, forKey: .sectionLayoutType)
        sectionItems      = try container.decode([GenericItem].self, forKey: .sectionItems).map { item in
        switch item.item_type {
        case .foo:
            // It's OK to force unwrap here because we already
            // know what type the item object is
            return Foo(id: item.id, itemType: item.item_type, title: item.title, imageURL: item.feature_image_url, audioURL: item.audio_url!)
        case .bar:
            return Bar(id: item.id, itemType: item.item_type, title: item.title, imageURL: item.feature_image_url, videoURL: item.video_url!, director: item.director!)
        }
    }
}

Usage:

let sections = try JSONDecoder().decode(Sections.self, from: json).sections

Correctly Parsing JSON in Swift 3

First of all never load data synchronously from a remote URL, use always asynchronous methods like URLSession.

'Any' has no subscript members

occurs because the compiler has no idea of what type the intermediate objects are (for example currently in ["currently"]!["temperature"]) and since you are using Foundation collection types like NSDictionary the compiler has no idea at all about the type.

Additionally in Swift 3 it's required to inform the compiler about the type of all subscripted objects.

You have to cast the result of the JSON serialization to the actual type.

This code uses URLSession and exclusively Swift native types

let urlString = "https://api.forecast.io/forecast/apiKey/37.5673776,122.048951"

let url = URL(string: urlString)
URLSession.shared.dataTask(with:url!) { (data, response, error) in
  if error != nil {
    print(error)
  } else {
    do {

      let parsedData = try JSONSerialization.jsonObject(with: data!) as! [String:Any]
      let currentConditions = parsedData["currently"] as! [String:Any]

      print(currentConditions)

      let currentTemperatureF = currentConditions["temperature"] as! Double
      print(currentTemperatureF)
    } catch let error as NSError {
      print(error)
    }
  }

}.resume()

To print all key / value pairs of currentConditions you could write

 let currentConditions = parsedData["currently"] as! [String:Any]

  for (key, value) in currentConditions {
    print("\(key) - \(value) ")
  }

A note regarding jsonObject(with data:

Many (it seems all) tutorials suggest .mutableContainers or .mutableLeaves options which is completely nonsense in Swift. The two options are legacy Objective-C options to assign the result to NSMutable... objects. In Swift any variable is mutable by default and passing any of those options and assigning the result to a let constant has no effect at all. Further most of the implementations are never mutating the deserialized JSON anyway.

The only (rare) option which is useful in Swift is .allowFragments which is required if if the JSON root object could be a value type(String, Number, Bool or null) rather than one of the collection types (array or dictionary). But normally omit the options parameter which means No options.

===========================================================================

Some general considerations to parse JSON

JSON is a well-arranged text format. It's very easy to read a JSON string. Read the string carefully. There are only six different types – two collection types and four value types.

---

The collection types are

• Array - JSON: objects in square brackets [] - Swift: [Any] but in most cases [[String:Any]]
• Dictionary - JSON: objects in curly braces {} - Swift: [String:Any]

The value types are

• String - JSON: any value in double quotes "Foo", even "123"or "false" – Swift: String
• Number - JSON: numeric values not in double quotes 123 or 123.0 – Swift: Int or Double
• Bool - JSON: true or false not in double quotes – Swift: true or false
• null - JSON: null – Swift: NSNull

According to the JSON specification all keys in dictionaries are required to be String.

---

Basically it's always recommeded to use optional bindings to unwrap optionals safely

If the root object is a dictionary ({}) cast the type to [String:Any]

if let parsedData = try JSONSerialization.jsonObject(with: data!) as? [String:Any] { ...

and retrieve values by keys with (OneOfSupportedJSONTypes is either JSON collection or value type as described above.)

if let foo = parsedData["foo"] as? OneOfSupportedJSONTypes {
    print(foo)
} 

---

If the root object is an array ([]) cast the type to [[String:Any]]

if let parsedData = try JSONSerialization.jsonObject(with: data!) as? [[String:Any]] { ...

and iterate through the array with

for item in parsedData {
    print(item)
}

If you need an item at specific index check also if the index exists

if let parsedData = try JSONSerialization.jsonObject(with: data!) as? [[String:Any]], parsedData.count > 2,
   let item = parsedData[2] as? OneOfSupportedJSONTypes {
      print(item)
    }
}

---

In the rare case that the JSON is simply one of the value types – rather than a collection type – you have to pass the .allowFragments option and cast the result to the appropriate value type for example

if let parsedData = try JSONSerialization.jsonObject(with: data!, options: .allowFragments) as? String { ...

Apple has published a comprehensive article in the Swift Blog: Working with JSON in Swift

---

===========================================================================

In Swift 4+ the Codable protocol provides a more convenient way to parse JSON directly into structs / classes.

For example the given JSON sample in the question (slightly modified)

let jsonString = """
{"icon": "partly-cloudy-night", "precipProbability": 0, "pressure": 1015.39, "humidity": 0.75, "precip_intensity": 0, "wind_speed": 6.04, "summary": "Partly Cloudy", "ozone": 321.13, "temperature": 49.45, "dew_point": 41.75, "apparent_temperature": 47, "wind_bearing": 332, "cloud_cover": 0.28, "time": 1480846460}
"""

can be decoded into the struct Weather. The Swift types are the same as described above. There are a few additional options:

• Strings representing an URL can be decoded directly as URL.
• The time integer can be decoded as Date with the dateDecodingStrategy .secondsSince1970.
• snaked_cased JSON keys can be converted to camelCase with the keyDecodingStrategy .convertFromSnakeCase

---

struct Weather: Decodable {
    let icon, summary: String
    let pressure: Double, humidity, windSpeed : Double
    let ozone, temperature, dewPoint, cloudCover: Double
    let precipProbability, precipIntensity, apparentTemperature, windBearing : Int
    let time: Date
}

let data = Data(jsonString.utf8)
do {
    let decoder = JSONDecoder()
    decoder.dateDecodingStrategy = .secondsSince1970
    decoder.keyDecodingStrategy = .convertFromSnakeCase
    let result = try decoder.decode(Weather.self, from: data)
    print(result)
} catch {
    print(error)
}

Other Codable sources:

• Apple: Encoding and Decoding Custom Types
• HackingWithSwift: Codable Cheat Sheet
• Ray Wenderlich: Encoding and Decoding in Swift

Decode particular value for key of Data using JSONDecoder in Swift 4.0

Firstly declare struct of those types.

struct Root : Decodable {
    let status : String
    let totalResults : Int
    let articles : [Article]
}

struct Article : Decodable {
    {//some key value pairs},
    {//some key value pairs}
}

Suppose the json string is jsonStr.
Now convert this json into data.

let data = Data(jsonStr.utf8)

Now try to decode this data.

let decodedStruct = fromJSON(data)

Here is the definition of fromJSON() method

static func fromJSON(jsonData: Data) -> Root? {
     let jsonDecoder = JSONDecoder()
     do {
          let root = try jsonDecoder.decode(Root.self, from: jsonData)
          return root
     } catch {
          return nil
     }
}

How to decode a property with type of JSON dictionary in Swift [45] decodable protocol

With some inspiration from this gist I found, I wrote some extensions for UnkeyedDecodingContainer and KeyedDecodingContainer. You can find a link to my gist here. By using this code you can now decode any Array<Any> or Dictionary<String, Any> with the familiar syntax:

let dictionary: [String: Any] = try container.decode([String: Any].self, forKey: key)

or

let array: [Any] = try container.decode([Any].self, forKey: key)

Edit: there is one caveat I have found which is decoding an array of dictionaries [[String: Any]] The required syntax is as follows. You'll likely want to throw an error instead of force casting:

let items: [[String: Any]] = try container.decode(Array<Any>.self, forKey: .items) as! [[String: Any]]

EDIT 2: If you simply want to convert an entire file to a dictionary, you are better off sticking with api from JSONSerialization as I have not figured out a way to extend JSONDecoder itself to directly decode a dictionary.

guard let json = try JSONSerialization.jsonObject(with: data, options: []) as? [String: Any] else {
  // appropriate error handling
  return
}

The extensions

// Inspired by https://gist.github.com/mbuchetics/c9bc6c22033014aa0c550d3b4324411a

struct JSONCodingKeys: CodingKey {
    var stringValue: String

    init?(stringValue: String) {
        self.stringValue = stringValue
    }

    var intValue: Int?

    init?(intValue: Int) {
        self.init(stringValue: "\(intValue)")
        self.intValue = intValue
    }
}

extension KeyedDecodingContainer {

    func decode(_ type: Dictionary<String, Any>.Type, forKey key: K) throws -> Dictionary<String, Any> {
        let container = try self.nestedContainer(keyedBy: JSONCodingKeys.self, forKey: key)
        return try container.decode(type)
    }

    func decodeIfPresent(_ type: Dictionary<String, Any>.Type, forKey key: K) throws -> Dictionary<String, Any>? {
        guard contains(key) else { 
            return nil
        }
        guard try decodeNil(forKey: key) == false else { 
            return nil 
        }
        return try decode(type, forKey: key)
    }

    func decode(_ type: Array<Any>.Type, forKey key: K) throws -> Array<Any> {
        var container = try self.nestedUnkeyedContainer(forKey: key)
        return try container.decode(type)
    }

    func decodeIfPresent(_ type: Array<Any>.Type, forKey key: K) throws -> Array<Any>? {
        guard contains(key) else {
            return nil
        }
        guard try decodeNil(forKey: key) == false else { 
            return nil 
        }
        return try decode(type, forKey: key)
    }

    func decode(_ type: Dictionary<String, Any>.Type) throws -> Dictionary<String, Any> {
        var dictionary = Dictionary<String, Any>()

        for key in allKeys {
            if let boolValue = try? decode(Bool.self, forKey: key) {
                dictionary[key.stringValue] = boolValue
            } else if let stringValue = try? decode(String.self, forKey: key) {
                dictionary[key.stringValue] = stringValue
            } else if let intValue = try? decode(Int.self, forKey: key) {
                dictionary[key.stringValue] = intValue
            } else if let doubleValue = try? decode(Double.self, forKey: key) {
                dictionary[key.stringValue] = doubleValue
            } else if let nestedDictionary = try? decode(Dictionary<String, Any>.self, forKey: key) {
                dictionary[key.stringValue] = nestedDictionary
            } else if let nestedArray = try? decode(Array<Any>.self, forKey: key) {
                dictionary[key.stringValue] = nestedArray
            }
        }
        return dictionary
    }
}

extension UnkeyedDecodingContainer {

    mutating func decode(_ type: Array<Any>.Type) throws -> Array<Any> {
        var array: [Any] = []
        while isAtEnd == false {
            // See if the current value in the JSON array is `null` first and prevent infite recursion with nested arrays.
            if try decodeNil() {
                continue
            } else if let value = try? decode(Bool.self) {
                array.append(value)
            } else if let value = try? decode(Double.self) {
                array.append(value)
            } else if let value = try? decode(String.self) {
                array.append(value)
            } else if let nestedDictionary = try? decode(Dictionary<String, Any>.self) {
                array.append(nestedDictionary)
            } else if let nestedArray = try? decode(Array<Any>.self) {
                array.append(nestedArray)
            }
        }
        return array
    }

    mutating func decode(_ type: Dictionary<String, Any>.Type) throws -> Dictionary<String, Any> {

        let nestedContainer = try self.nestedContainer(keyedBy: JSONCodingKeys.self)
        return try nestedContainer.decode(type)
    }
}

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